The principal problem encountered in designing and operating effective gasketed connections is seal failure and the leakage associated therewith. Leakage from gasketed joints can be attributed to one or more of five root causes, including (a) sealing surfaces that do not meet the minimum physical conditions necessary to effect a seal, (b) insufficient gasket seating stress to effect a seal, (c) excessive gasket seating stress which results in damage to the sealing components, (d) a reduction in gasket stress due to flange rotation or in-service creep relaxation of the gasket sealing material, and (e) incompatibility of the gasket sealing material with the sealed fluid or the operating environment (which leads, over time, to gasket degradation).
Each of the aforementioned causes is typically regarded within industry as a failure of the gasketing material, but in reality only in case (e) is the gasketing material the source of the leak. In the remaining cases, connection life is reduced by the assembly practice, the joint design configuration, the joint condition, or some combination thereof, but the conventional approach has been to focus on the composition or configuration of the gasketing material to compensate for design or production flaws in the parts to be sealed.
Failures attributable to case (a) above, in which the gasket fails because of a sealing surface variance such as an out-of-flatness condition, misalignment of the sealing components, corrosion of the sealing surface, or scratches or other surface defects, are manageable by ensuring that the gasketing material is of a sufficient thickness (where possible) to accommodate the sealing surface variance and by adhering to effective manufacturing tolerances and effective maintenance practices. In the remaining cases (b) through (e), however, the design of components and selection of sealing materials are implicated.
Specifically, the gasket design and material selection effectively mitigates leakage events resulting from cases (b) through (e) when each of the following criteria is met (i) the gasket seating stress is effectively controlled; (ii) the gasket seating stress is evenly distributed over the entire gasket sealing area; (iii) a sealing material compatible with the environment is selected; and (iv) the in-service creep relaxation of the material is limited. Consequently, there is a need for a gasket that satisfies these design criteria.
Existing flat gasket designs generally fall into one of four basic groups: sheet gaskets, reinforced sheet gaskets, metallic gaskets, and spiral-wound gaskets. With sheet gaskets, which include gaskets made of rubber, compressed fiber, polytetrafluoroethylene (PTFE), graphite, and equivalents, the performance is limited, variously, by the primary sealing material's compression characteristics, the gasket's tendency to creep in service, the gasket's ability to withstand various combinations of temperature and pressure, and degradation of the gasketing material. Reinforced sheet gaskets arose in order to help mitigate blow out resistance and creep relaxation.
Metallic gaskets consist of a metal carrier ring upon either side of which a sealing material has been affixed, and further help to improve blow out resistance and to reduce creep relaxation. For example, in kammprofile gaskets, the profile is serrated and serves to reduce creep relaxation of the sealing material across the carrier ring, which providing concentric regions of increased gasket stress radially across the sealing surface. In other designs, the carrier ring extends radially outward beyond the sealing area, with provisions made to accommodate the fasteners in order to make use of the fasteners to align the gasket within the flange.
Spiral-wound gaskets include a central spiral-wound sealing component and an inner or outer ring (or both) that serves as a compression stop, and, in theory, represent the conventional pinnacle of sealability in terms of their blow out resistance, creep relaxation minimization, and limited compression. However, research and field experience have shown that most spiral-wound gaskets continue to be subject to creep relaxation and inward buckling of the metal windings, both of which potentially result in leakage. Only spiral-wound gaskets with both inner and outer rings tend to mitigate creep relaxation and winding buckling, although radial buckling of the inner ring has been noted in some applications.
A further problem associated with the use of spiral-wound gaskets involves standards within the manufacturing process; current manufacturing processes create variability, from gasket to gasket, in the density of the filler material—in some cases, a factor of as much as 3 in difference between similar gaskets. Thus, the repeatable achievement of a target gasket seating stress using spiral-wound gaskets is problematic. Further compounding this problem are assembly standards that allow a range of acceptable gasket compression; since gasket seating stress is directly correlated with gasket compression, spiral-wound gaskets are subject to differential seating stresses. Correspondingly, the compression stop rings in conventional spiral-wound gaskets serve only to limit the maximum compression of the gasket, rather than identifying the proper compression Moreover, where full compression does not exist, creep relaxation can occur.
Still another problem associated with the use of conventional gaskets is the need for calibrated assembly tools to ensure that the proper compression is achieved. It is important to note that the compression stops in existing gaskets provide only a maximum compression limit without regard to the proper compression required. Therefore, calibrated tools must be used with conventional gaskets in order to ensure that the correct compression is achieved.
Finally, existing gaskets are subject to variable seating stresses over their surface area due to the effect of flange rotation, which may be most pronounced in raised-face flanges. Flange rotation is a term used to portray the change in parallelism that occurs between the opposing flange faces as tightening of the joint occurs. The susceptibility to flange rotation varies according to component-specific factors that include flange size, material, and pressure class. For example, in raised-face flanges, once the opposing raised-faces each engage the gasket located in the central region of the flange, if additional preload is added to the fasteners, there is a tendency for the flange to pivot radially about the gasket, thus causing the flange faces to increase in relative proximity to one another as a function of distance from their axial centerline. This results in non-uniform gasket seating stresses over the sealing area, since the gasket seating stress increases radially from inside to outside due to the flange rotation. Consequently, required bolt preloads calculated to yield a desired gasket stress on assembly, may achieve that target gasket seating stress only at the outer circumference of the seating area, with the actual seating stress decreasing below the target seating stress as you move radially inward over the sealing area due to flange rotation.
What is needed, therefore, is a gasket whose design satisfies the aforementioned criteria and provides for the achievement of a correct, uniform gasket seating stress without the use of calibrated assembly tools, and for the maintenance of that stress over time.